Only a portion of oil present in an oil-bearing formation is recoverable as a result of the natural pressure of the formation. The oil recovered from this “primary” recovery typically ranges from 5% to 35% of the oil in the formation. Enhanced oil recovery methods have been developed to increase the amount of oil that may be recovered from an oil-bearing formation above and beyond that recovered in primary recovery.
Water-flooding, in which water is injected through an injection well into an oil-bearing formation to mobilize and drive oil through the formation for production from a production well, is a widely used method of secondary recovery used to increase the amount of oil recovered from a formation beyond primary recovery. Recently, water-flooding utilizing water having low salinity has been utilized to increase the amount of oil recovered from a formation relative to the amount of oil recovered in a conventional higher salinity water-flood. Low salinity water may be used in place of higher salinity water conventionally used in a water-flood in a secondary recovery, or low salinity water may be used after a conventional higher salinity water-flood to incrementally increase oil recovery over that of the initial water-flood in a tertiary recovery process.
Low salinity water utilized in low salinity water-flooding has a lower ionic strength than connate water present in the formation, typically having an ionic strength of 0.15 M or less and having a total dissolved solids (“TDS”) content of from 200 parts per million (“ppm”) to 10,000 ppm and a multivalent cation content that is less than the multivalent cation content of the connate water. Injection of low salinity water into a formation may reduce the ionic bonding of oil to the formation within pores in the formation by double layer expansion, leading to a reduction in the rock's adsorption capacity for hydrocarbons. This increases the mobility of the oil in the formation by making the surface of the pores of the formation more water-wet and less oil-wet, permitting the mobile oil to be removed from the pores in which it resides and to be driven to a production well for production from the formation.
In an enhanced oil recovery process utilizing water-flooding, oil and water, and typically gas as well, are produced from the formation together. The oil, water, and gas are separated in a separator to recover the oil from the produced water and gas. Free water is separated and removed from the oil by phase separation. At least a portion of the oil and a portion of the water, however, may be intimately mixed in an emulsion. The emulsion may be treated in a coalescer that helps break the emulsion by causing water in the emulsion (in a water-in-oil emulsion) or oil in the emulsion (in a oil-in-water emulsion) to coalesce and phase separate. The separated phases may then be recovered separately.
Improved processes for separating oil and water produced from an oil-bearing formation by a low salinity water-flood enhanced oil recovery process are desirable.